Page 49 - MetalForming January 2014

Tooling by Design
Wrinkles in the flange area or near the die radius indicate the presence of in-plane compressive forces synony- mous with deep drawing. Blankholder pressure may need to increase to as much as 40 percent of punch force, depending on material type and thick- ness. Reducing the size of the die entry radius (< 10t), adding draw beads or reducing punch-to-die clearance also may help.
Punch-to-die clearances in stretch- ing operations generally are equal to material thickness (1t). In drawing operations, compressive forces in the flange cause the material to thicken, requiring additional clearance of 10 to 25 percent or more, depending on material type.
When fracturing of the sheetmetal occurs, the slide position will differ depending on the forming mode. Drawing failures generally occur with- in the first third of the draw depth (see image). Forming loads and friction are greatest early in the stroke and dimin- ish in magnitude as the blank area diminishes. Conversely, stretching fail- ures generally occur near the end of the forming stroke due to excessive thin- ning and work hardening.
Venting
Most tool and die professionals understand the need to vent draw dies. Venting allows trapped air and lubri- cants to escape and prevents vacuum problems (part sticking) on the upstroke.
However, venting also is important for stretching operations to prevent trapped air and fluids from causing bursting or fracturing in highly stretched areas. Unfortunately, after breakage occurs the fluid and air are released, taking with them any evi- dence of the root cause of the failure. The stamper likely will waste time and effort on tool modifications and material-specification changes, when the real culprit was trapped air and fluid. MF
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